1. Field of the Invention
The present invention relates to a repeatedly rewritable image display medium using particles, and an image formation apparatus.
2. Description of the Related Art
Conventionally, as repeatedly rewritable image display media, there are proposed display techniques such as Twisting Ball Display (two-colored rotating-particle display), electrophoresis, magnetophoresis, thermal rewritable medium, and liquid crystal having a memory property. Although such display technologies are excellent in memory property of an image, they are not capable of providing a paper-white display on a screen, thereby causing a problem of low density contrast.
On the other hand, as the display technique using a toner for overcoming the foregoing problem, there is proposed the following display technique. Namely, a conductive colored toner and white particles are sealed between opposing electrode substrates. Thus, charges are injected to the conductive colored toner through a charge transport layer disposed on the inner surface of the electrode substrate on the non-display side. The charge-injected conductive colored toner is then moved to the electrode substrate on the display side, situated opposing to the electrode substrate on the non-display side in response to the electric field applied between both the electrode substrates, and adheres to the inner side of the electrode substrate on the display side to provide image display by contrast between the conductive colored toner and the white particles (Japan Hardcopy"" 99 Paper Collection, pp.249-252). With this display technique, the whole image display medium is constituted of a solid, so that the display can be advantageously switched between white and black (colors) 100% in principle. However, the foregoing technique has the following problem. That is, there occur a conductive colored toner in no contact with the charge transport layer disposed on the inner surface of the electrode of the non-display substrate, and a conductive colored toner isolated from other conductive colored toners. These conductive colored toners are not injected with charges, and hence they are not moved in response to the electric field. Accordingly, they are randomly present between both the electrode substrates, resulting in a reduction in density contrast.
The present inventors have proposed an image display medium using particles, excellent in density contrast, which includes a pair of substrates, and plural types of particle groups sealed movably in response to the applied electric field between the substrates, and having different colors and charging characteristics (Japanese Patent Application No. 2000-165138). According to this proposal, it is possible to obtain a high whiteness degree and density contrast. With the constitution of the particles in this proposal, the white density, black density, and density contrast are excellent at an initial stage, but when rewriting is performed repeatedly for a long period, the image density may be reduced to reduce the density contrast, or the uniformity of the image may be reduced to cause image irregularity.
The present invention has been made in view of the above circumstances and provides an image display medium which is capable of displaying an image which shows little change in image density, and little change in density uniformity, and has a stable density contrast even when rewriting is carried out repeatedly over a long period, and an image formation apparatus.
A close study has proved that the foregoing problem is caused by destabilization of the charging amount and broadening of charge distribution (charging distribution) due to frictional charging between particles. Then, the inventors have found that it is effective to appropriately set the value of the coefficient of variation in the particle size distribution of particles to be used in order to stabilize the charging amount due to frictional charging between particles. Consequently, they have completed the present invention.
According to an aspect of the present invention, an image display medium has a pair of substrates disposed opposing to each other; and particle groups including at least two types of particles sealed in a space between the pair of the substrates, at least one type thereof having a positively chargeable property, and at least one of others having a negatively chargeable property, and the positively and negatively chargeable types of particles being of different colors. The value of a coefficient of variation in each particle size distribution of both the positively and negatively chargeable types of particles is approximately not more than 15%.
In the present invention, the positively and negatively chargeable types of particles may be of different colors, and that the particle size distributions thereof are respectively sharp distributions. A difference in color therebetween provides a density contrast between the image area made up of the positively chargeable particle group and the image area made up of the negatively chargeable particle group. Further, by sharpening the particle size distributions of both the positively and negatively chargeable types of particles, it is possible to improve the speed of the responsibility among particles between the pair of the substrates. Still further, since the collision efficiency between particles is improved, it becomes possible to stabilize frictional charging and make the charging distribution narrow.
In the image display medium of the present invention, it is desirable that one type of the positively and negatively chargeable types of particles are of white color. By setting at least one type of particles to be of white color, it is possible to improve the tinting strength of the particles, and the density contrast. Further, it is desirable that the white color particles contain a coloring material and that the coloring material is titanium oxide. By using titanium oxide as the coloring material, it is possible to increase the hiding power in the range of visible light wavelengths, and still further improve the density contrast.
In the image display medium of the present invention, it is desirable that at least one of the positively chargeable type of particles and the negatively chargeable type of particles is the type particles obtained by moving an oil phase in which components making up the particles have been dissolved and/or dispersed to an aqueous phase through a film having pores for granulation.
The particles manufactured in the foregoing manner are obtained from granulation process by moving the oil phase to the aqueous phase through the film having pores, and thereby forming oil droplets. Therefore, the diameter of each particle is controlled by the diameter of each pore of the film. As a result, it is possible to sharpen the particle size distribution of the obtained particles only by controlling the pores of the film. Accordingly, it is possible to manufacture the particles with the particle size distribution specified in the present invention with ease, so that the effect of the invention can be achieved with ease and at a high level.
According to another aspect of the present invention, an image formation apparatus which forms an image on the image display medium of the present invention described above has an electric field generation part which generates an electric field corresponding to an image between the pair of the substrates.